Semiconductor fabrication photolithography processes have used light beams, electron beams, x-rays, and ion beams to expose fine patterns of a mask on a photoresist film on the doped surface of a thin semiconductor wafer substrate. The photoresist is then developed with a liquid solvent that removes the unexposed (or exposed) photoresist. The wafer is then etched to form the pattern. Successive processes like this use other masks to expose other fine patterns on new films of photoresist to form integrated circuits layer by layer on the wafer substrate. This process is termed photolithography.
In general, such photolithography processes are wet processes which require the semiconductor wafer to be exposed to liquid chemicals under controlled conditions. Moreover, several steps are typically required to form a desired pattern.
Photochemical reactions at the surface of a semiconductor can also be induced by a laser beam which heats the semiconductor to induce the chemical reaction. Ablative photodecomposition is a phenomenon observed at the surface of a metallic film to which laser pulses are directed and for which the fluence of a pulse exceeds a threshold. As a result, the products of ablative photodecomposition are ejected from the surface of the film, or "ablate". These ablated particles may then be deposited under the influence of a vacuum onto a target substrate. One such laser process is termed laser ablation deposition (LAD).
With laser ablation deposition (LAD), a desired metal pattern may be directly written on a target substrate without a photolithography process. This process is also termed as metal scribing or writing.
FIG. 1 is a schematic diagram of a representative prior art apparatus for performing a laser ablation deposition (LAD) process. In general, with this process a transparent support substrate 10 having a thin metal film 12 deposited thereon is situated in close proximity to a target substrate 14. The transparent support substrate 10 and target substrate 14 are mounted in a sealed process chamber 16 which is subjected to a vacuum source 18. A laser light source 20 directs a laser light beam 22 through a focusing lens 24 and through a quartz window 26 into the process chamber 16. The laser light beam 22 is focused upon the transparent support substrate 10 to ablate the thin metal film 12 along a desired pattern 28.
The laser beam is sufficient to produce a required intensity at the target (pulse fluence, in Joules/cm.sup.2) and to melt the metal film 12 through the underlying transparent support substrate 10. The metal is then forward ablated from the transparent support substrate 10 onto the target substrate 14 in a desired pattern 30.
There are several limitations with this prior art laser ablation deposition method. In general, the ablation and deposition process is difficult to control because the transparent support substrate 10 must be formed of a transparent material such as quartz which typically has a low thermal conductivity. It is thus difficult to control the heating of the metal film 12 by the support substrate 10 to the laser fluence threshold necessary for ablating the film 12 onto the target substrate 14. In general, this process (LAD) is thus not adaptable to large scale semiconductor manufacturing.
The present invention is directed to a simple yet unobvious laser ablation deposition (LAD) method that allows the ablation of a metal film on a substrate to be closely controlled. This allows the process to be adapted to large scale production of semiconductor wafers.